Broadcast television in the United States is broadcast in accordance with the National Television Standards Committee (NTSC) broadcast television format standard. The NTSC broadcast format provides for the transmission of approximately thirty image frames per second. A particular frame includes two interlaced fields, in which each field includes 265½ horizontal scan lines. FIG. 1 illustrates an example television broadcast frame 10 in accordance with the NTSC standard. Broadcast frame 10 may also be referred to as a “broadcast image.”
The broadcast and display of frame 10 is described in connection with a conventional analog television having a cathode ray tube (CRT) with an electron beam. A receiver portion of the television receives the broadcast NTSC signal and controls the electron beam such that the electron beam is scanned across the screen of the CRT to generate the appropriate image. The first nine horizontal scan lines (lines 1-9) of the frame 10 are used to synchronize the television receiver and to return the electron beam to the top of the screen. The electron beam is disabled (also referred to as “blanked”) during the scan time of the first twenty-one scan lines so that the electron beam does not generate a visible line from the bottom of the screen to the top of the screen when being returned to the top of the screen. The first twenty-one scan lines are commonly referred to as the “vertical blanking interval” lines (or VBI lines).
The odd scan lines of the frame (i.e., frame line 1, frame line 3, etc.) are received first and are referred to as the “odd field”. The first twenty-one of these odd lines are the VBI lines. These first twenty-one lines synchronize the television receiver for the subsequent scanning of the horizontal scan lines of a viewable portion 12 of the frame 10. During each horizontal scan line, the electron beam scans from left to right across the screen. When the electron beam reaches the right edge of the screen, the electron beam is returned to the left edge of the screen in preparation for the scanning of the next scan line. After the scanning of each odd scan line in viewable portion 12, the electron beam is “blanked” as the electron beam is returned to left edge of the screen in preparation for the start of the next scan line. This blanking time is referred to as the “horizontal blanking interval” of the frame, which is indicated by a region 14 in FIG. 1.
After the last odd scan line has finished, the even scan lines of the frame (i.e., frame line 2, frame line 4, etc.) are received and are referred to as the “even field”. As with the odd field discussed above, the first twenty-one scan lines of the even field are VBI lines. The electron beam is blanked during the scanning of the even VBI lines such that the electron beam can be returned to the top of the screen without generating a line on the screen. After the scanning of all the even VBI lines, the even scan lines of viewable portion 12 are scanned in a manner similar to the scanning of the odd scan lines discussed above. The viewable horizontal scan lines of the odd and even fields together cause the electron beam to scan across the screen of the television to create the viewable television image.
Generally, the entire vertical blanking interval is not required for vertical synchronization and returning the electron beam to the top of the screen. Typically, only VBI lines 1-9 of each field (i.e., the odd field and the even field) are used for these purposes. Thus, the remaining twelve VBI lines (lines 10-21) are available for communicating other information.
For example, VBI line 21 provides a low speed communication sub-channel (the “line 21 sub-channel”). Data is encoded in the line 21 sub-channel in accordance with the encoding technique set forth in EIA-608, which is reserved by the Federal Communication Commission (FCC) for captioning services. The line 21 sub-channel can transmit textual information, the current time, “V-chip” program rating codes, and closed captioning information. For example, a television receiver captures the closed captioning information and displays it as a text subtitle along with the television video to assist the hearing impaired.
VBI lines 10-20 are available for a higher-speed communication sub-channel for the communication of data. Data can be encoded in VBI lines 10-20 at a data rate sufficient to communicate web page content along with the television content being transmitted. The web page content is received and stored locally in the television receiver for future access. Later, when the web page content is to be displayed at a particular time in the television video, a “trigger” that identifies the web page content (e.g., by file name) is broadcast to the television receiver. When the trigger is received, the associated web page content is retrieved from the local storage device and displayed on the television screen.
However, certain televisions do not support the transmission of data over the VBI lines as discussed above. Furthermore, certain broadcast systems (such as digitally compressed satellite feeds and digital cable systems) may prevent the VBI-based information from being delivered to capable receivers. Thus, an alternate system sends information and triggers using line 21 instead of the VBI lines. Typically, the information transmitted using line 21 is relatively short, such as a uniform resource locator (URL) identifying the location of web page content to be retrieved by a television receiver (e.g., via a telephone line or other network connection). Since the television broadcaster transmits the same signal to many televisions, the broadcaster cannot tailor its signal to match the capabilities of a particular television receiver.
The system described herein addresses these and other problems by providing a television broadcast method and system that supports the transmission of data to various types of television receivers.